en 1.8 /summon Game Mods Featuring: Backpack
Vanilla backpack! This backpack features 15 slots and can carry anything!

How it works:
1.Hold out a Chest named "Backpack".
2.Right-click.
3.Press 'E' to close the backpack.

Hope you like it, enjoy!
     -CarmineKing

I got the idea while playing modded worlds. I then came up with the concept of:
A mule will teleport to a player holding a 'backpack', then return to its hold when the 'backpack' is no longer held.

PLEASE Feel free to give me some suggestions in the comments! I will do my best to make a command for it.